Art of manufacturing spark-plugs.



- P. M. FURBER.

ART OF MANUFACTURING SPARK PLUGS.

Patented Feb.2, 1915.

APPLICATION FILED JULY 21,1914.

LI Ill/421 azz a,

' mas Pr-ii FREDERICK 1V1. FURBER, OF REVERE, MASSACHUSETTS, ASSIGNOR T0 ARTHUR R. MOSLER, OF NEW. YORK, N. 'Y.

ART OF MANUFACTURING SPARK-PLUGS.

Specification of Letters Patent.

Patented Feb. 2, 1915.

Original application filed July 5, 1913, Serial No. 777,578. Divided and this application filed July 21, 1914.

I Serial No. 852,296.

of Revere, in the county of Sufi'olk and State of Massachusetts, have invented a new and useful Improvement in the Art of Manufacturing Spark-Plugs, of which the following is a specification.

This invention relates to methods of manufacturing spark plugs of the character commonly used in internal combustion engi'nes.

Spark plugs usually comprise an outer body or shell adapted to be secured in the cylinder of an engine, an insulator, a pair of electrodes electrically separated by the in 's'ulator, and some means for clamping'the insulator in the shell. The insulators used in these devices are necessarily made'of material that renders them comparatively fragile and the frequent breakage of these parts is one of the chief elements of trouble experienced in operating engines of this type. This result is caused partly by the fact that the plugs are subjected to very rough usage, but is due in a large measure to the manner in which the insulators are secured in their shells.

The present invention has for its object to improve the method of manufacturing spark lugs, with a view particularly to reducing he manufacturing cost and to producing a more substantial serviceable plug.

The invention will be readily understood from the following description of one embodiment thereof, reference being made to the accompanying drawings, in which Figure l is a view in side elevation of the preferred form of the invention; Fig. 2 is a longitudinal central sectional View of the plug shown in Fig. 1; Fig. 3 is a View in side elevation of the insulator used in the lug and also shows in crosssection the busiiing by which it is secured in operative position; Fig. 4 is a cross sectional view through the insulator and bushing at one stage in the manufacture of the device; and Fig. 5 is a cross sectional 'view similar to Fig. fl, 'butshowing the parts-at a later stage 1n the sired diameter the lower part of a piece of stock of the required length, forming a threaded portion 4 on this reduced part and a plain sleeve-like portion 6 belowthe threaded part 4. The shell is then drilled and reamed to form the bore 8, this bore preferably being cylindrical in shape-from its lower end up to a point indicated approximately at a, Fig. 2, and then tapering gradually from this point to the outer end of the shell. This taper is very slow or gradual and is of the character sometimes termed a sticking taper. It will be evident that this shell can be formed automatically in a turret lathe, all the operations being performed before the shell is severed from the bar from which it was made, thus requiring but a single handling of this member.

The insulator 10 is formed with a tapered exterior conforming to the tapered bore of the shell and is held in operative position in the shell by means of a thin bushing 12 which is fitted tightly on to the tapered surface of the insulator, the insulator and the bushing fixed thereon then being forced into the tapered bore of the shell where the frictional engagement of the bushing with the insulator and shell holds these parts in their proper relationship.

Unless unusual care is taken infthe manufacture of the insulators commonly used in spark plugs, they will not fit accurately the seat in the shell for which they are intended.

Most of these insulators are made of porce- I lain and this material warps in the bakingoperation; but, even if other materials are used, more than ordinary care must be exercised and special operations must be per-' formed, thus involving additional expense, in order to make the insulators fit the shells properly. For this reason it has been usual to secure the insulators in their shells with flexible packings but this expedient is open to the objection that it-produces an unequal distribution of pressure on the insulator and these inequalities are intensified by the sudden changes in temperature to which the plug is subjected. In fact, this manner of holding the insulator and the consequent unequal distribution of strain in the insulator is one of the chief causes of the frequent breakage of these parts.

A close examination of any considerable number of insulators of the type shown in centage ofthem have neither'a true tapered surface nor a cross sectionaloutline of exactly circular form. Fig. 4: shows a cross sectional outline that one might reasonably expectto find ,in an insulator of this type. Where it is necessar or desirable to use .insu atqrssubiectitathesexvariations, I we for to secure the insulator in the shell by the following metod: Assuming a shell to have been; completed", and: "an insulator of the proper form a d made in the usual manner having beenprovided, the bushing 12 is next :placed ona the insulator; in the position shownzin full .linesiin; Fig. 3, and is then forced into the dotted ,line position shown in .rFig. 3 by any convenient mechanism, as, for instance, by means of'a suitable die.- .This "bushing may be cut from tubing ofmproper 'size. and preferably-is madeof some soft metal, such, for instance, as aluminum or copper. In being forced on to the insulator in;the manner just described, the bushing will bedistorted in accordance with the external shape .of the.- insulator. and will be -.causedto conform to the irregularities of tli,e"surface of the insulator; It will be noted that the bushingis forced on to a part of; the insulator the smallest. diameter of whichis greaterthan the original diameter 50f the bushing so that a tight fit between the tapered surface of the insulator and the inner surface of the bushing is insured at all points. A transverse section through the insulatorand bushing at this time would appear substantially as shown in Fig. 4, the exterior of thelbushing being somewhat eccentric .due to the irregularities introduced into it by the shape of the insulator.

The insulator with the bushing fast there- 'on is next machined in a suitable lathe or jig, or is acted upon in any other convenient manner, to shape its outer surface to fit the tapered seat in the shell 2. By this operation, .i'a-true surface is given to the bushing 12, and a-Htransverse section through the bushing and insulator after this operation forming a shell with a bore gradually tahas been completed would appear substantially as shown in Fig. 5. Next,'the insulator and the bushing fast thereon are forced into the bore .8 of the shell 2 with sufficient pressure to cause the frictional engagement between the bushing and the shell to hold the insulator securely in the; shell. The bushing thus has a gas tight fit both with the insulator and with the shell and it is so shaped in cross section, both transversely and longitudinally, that it compensates'for any variations in shape between the insula- -tor and the seatfor the insulator in the shell. A soft metal bushing, such, for instance, as aluminum. preferably is used since it conforms readily to the shape-of the insulator, it'is easily machined and itahasa tion that is gripped by the bushing and produces a substantially uniform distribution of pressure through the insulator. The bushing also acts somewhat as a cushion :for the insulator and relieves to some extentthe strains placed upon the insulator due to sudden changes in temperature. It will be observed that holding the insulator in the shell solely by means of a'friction bushing in the manner herein describel avoidsjathe use of devices bearing upon the insulator that can be manipulated by the user of the plug to increase the pressure on the insulator with the consequent liability of-breakmg it.

' he electrodes employed may be of an suitable form. In the construction shown, the sleeve-6 of the shell 2 constitutes .One electrode. The other electrode 14 is made of any of the common composition or alloys capable of resisting the disintegrating action of the spark, and is swaged into an electrode stem 16 that projects through a central bore in the insulator 10. An enlargement 18 of this stem rests in a suitable seat formed in the inner end of the insulator (that is, the end toward the cylinder) and the outer end of this stem is threaded to receive a threaded cap 20 and a binding nut 22. A flexible washer 24 preferably is interposed between the top of the insulator and the cap 20.

What I claim is:

1. That improvement in the art of manu- I facturing spark plugs which consists in providing a shell having a seat for an insulator, fitting a metal bushing tightly on said insulator to form a gas tight joint therewith, next shaping said bushing to cause it to conform to the seat in said shell, Y

- bushing fast on said insulator to form a gas tight joint therewith, and forcing said insulator and bushing into said bore with sullicient pressure to cause the frictional engagement of said bushing with said shell to hold the. bushing and insulator accuratelv in operative position in the shell.

3. That-improvement in the art ofmanufacturing spark plugs which consists in forming a shell with a bore gradually tapered toward its outer end, providing an lllSlllfliJOl Wltll-z a tapered surface approximately like that of said bore, forcing a soft metal bushing on to the tapered surface of said insulator with suificient pressure to cause it to conform to the tapered surface of said insulator and to form a gas tight joint therewith, machining the outer face of said bushing to cause it to fit the tapered bore of said shell, and then forcing said insulator and bushing into said bore with suflicient pressure to cause the friclo tional engagement of said bushing with I said shell to hold the bushing andinsulator securely in operative position in the shell. In testimony,- that I claim the foregoing as my invention, I have signed my name in presence of two witnesses, this sixteenth day 15 

